Yeah, it's only mentioned in a single sentence on the site I believe (looking for it now) but it really exists. My sister is currently paying off her 6S with this method.
Edit: Its mentioned on this page towards the bottom:
http://www.apple.com/shop/iphone/iphone-upgrade-program
iPhone Installments is what I'm referring to. It *is* only available at the physical stores though, but if you're near one just let an employee know you want the Installment plan and NOT the Upgrade plan.